1. The act of behaving like a ninja.
2. Thievery.
3. (less frequently) Any act of skulduggery.

1. With these new darkvision goggles I got in the mail, I can see the darkies sneaking around near the park. Their ninjary will get them nowhere!
2. The old lady isn't watching her purse, so quit being a pussy bitch and show me your ninjary skills.